There's not really the same differences that existed between browsers these days. The way things are headed it won't be long before they're all roughly the same speed. Curently I don't think there's a killer feature present on one, but not the other, the main differences are in the speed they render pages, and handle script etc.

Sadly Firefox chugs along very fast, until it comes time to use one of the many essential plug ins - Java, PDF, Flash, Silverlight etc. That's where things start to chug, and changing broswer would have no effect.